1. Field of the Invention
The present invention relates to an electromagnetic flow meter, and more particularly to an electromagnetic flow meter provided with a measuring pipe to flow fluid to be measured in which a resin lining portion is lined by pressure molding.
2. Description of the Background
An electromagnetic flow meter generates a magnetic field in a measuring pipe by flowing current in a coil, and detects a magnitude of an electromotive force generated according to the dielectric constant of the fluid flowing in the magnetic field so as to measure a flow rate. At the inner face of the measuring pipe of such an electromagnetic flow meter, the lining is performed for insulation, by resin such as fluorine contained resin and polyurethane resin and so on.
As a resin lining method of the electromagnetic flow meter, a method called transfer molding is known, for example. In this method, a die is fitted on both ends and an inner portion of the measuring pipe, and these are heated to an approximately melting point of the resin to be lined, and then the melted resin is pressurized and inpoured in gaps between the measuring pipe and the die.
In addition, as another resin lining method, a method called mold forming or injection molding is known. In this method, a die is fitted on both ends and an inner portion of the measuring pipe, and these are heated but not to an approximately melting point of the resin to be lined, and then the melted resin is pressurized and inpoured in gaps between the measuring pipe and the die.
Further, as another resin lining method, a method is known further in which a die is fitted on both ends and an inner portion of the measuring pipe, pellets of the resin to be lined are preliminarily inserted in gaps between the measuring pipe and the die, these are heated till the resin pellets to be lined melt in the gaps between the measuring pipe and the die, and then the resin is pressurized and molded.
In addition, hereinafter, these three molding methods are called generally as pressure molding methods.
In a conventional electromagnetic flow meter with a measuring pipe in which resin lining is performed, so as to prevent a resin lining portion from peeling off from the measuring pipe in such a case that the inside of the measuring pipe becomes at a negative pressure for the ambient air, a technique is known that the resin is inpoured for lining after a cylindrical plate with openings is pasted on the inner face of the measuring pipe as a reinforcing member.
A following forming method is disclosed (refer to Patent Document 1, for example). That is, a forming method is disclosed in which a tubular body with openings is formed by winding a plate with openings as a reinforcing member to be embedded in the resin in a cylindrical shape so as to be inserted in the measuring pipe, and then spacers are fitted at the outer circumference face of the tubular body with openings, and after the tubular body with openings is inserted and fitted in the measuring pipe, the measuring pipe is coated with resin by mold forming. In the forming method, before mold forming the tubular body with openings, while contacting both ends of the plate with openings tightly in the circumference direction, the plate with openings is displaced in the axis line direction so as to increase the diameter of the tubular body with openings till the outer circumference faces of the spacers contact tightly with the inner circumference face of the measuring pipe to form the tubular body with openings.
In addition, as a technique to prevent a resin lining portion from peeling off from a measuring pipe without using a reinforcing member, a technique is known that dovetail grooves are formed in circumference shapes at the inner face of the measuring pipe, or dovetail grooves are formed at the inner face of the measuring pipe in the axis direction, and the resin is inpoured for lining (refer to Patent Document 2, for example).
Patent Document 1: Japanese Patent Publication (Kokoku) PH3-124
Patent Document 2: Japanese Utility Model Publication (Kokoku) US61-16493
A forming method of a resin lining of a conventional electromagnetic flow meter will be described with reference to FIGS. 8A, 8B, 8C. FIG. 8A is a diagram to explain a lining structure, and FIG. 8B is a perspective view showing an appearance of a lining locking frame 27 to prevent the lining from peeling off.
A side sectional view of an upper portion from a center of a pipe axis of a measuring pipe 1 in FIG. 8A indicates a state before resin lining. The measuring pipe 1 to flow fluid to be measured is composed of a pipe body 1a, a pair of flanges 1b provided at the end portions of the pipe body 1a, and a pair of internal substance storing boards 1c to store the loading substance such as a coil and so on, which are welded on the outer circumference of the pipe body 1a symmetrically to the center of the pipe axis in the pipe axis direction of the pipe body 1a. 
In addition, a side sectional view of a lower portion from the center of the pipe axis in FIG. 8A indicates a state after resin lining to the measuring pipe 1. A resin lining portion 3 to be lined at the inner wall of the measuring pipe is made of an insulating material such as fluorine contained resin and polyurethane resin and so on. In addition, the lining locking frame 27 formed in a cylinder shape of a thin steel plate with a plurality of openings bored is embedded by mold forming.
The lining locking frame 27 is composed of a cylindrical plate with openings 2a and a plurality of belt-like ring plate 2b2 welded around the outer circumference of the cylindrical plate with openings 2a. 
The lining locking frame 27 is loaded in the measuring pipe 1 so as to contact the outer face with the inner face of the measuring pipe 1.
Then, a lining molding die not shown is fitted around the measuring pipe 1 loaded with the lining locking frame 27. And the resin lining material is pressure injected from an end of the measuring pipe 1 between the lining molding die and the inner face of the measuring pipe 1. By this, the resin lining material is filled around the lining locking frame 27 via a lot of openings A of the lining locking frame 27, so that the resin lining portion 3 is formed by embedding the lining locking frame 27.
The resin lining portion 3 formed as described above is provided with a structure in which the resin lining portion 3 is locked by the lining locking frame 27 and is enforced, and in addition, the lining portion 3 is made possible to outstand the change in negative pressure and external pressure and is not peeled off easily from the measuring pipe 1.
Next, a problem of forming the resin lining portion 3 of the conventional electromagnetic flow meter constructed as described above will described with reference to FIG. 8C.
Regarding the lining locking frame 27, the belt-like ring plates 2b2 are contacted with, and a few of the belt-like ring plates 2b2 are spot welded to the inner circumference face of the measuring pipe 1. In this case, there is a problem that the diameter of the cylindrical plates with openings 2a made of the thin steel plate and the inner diameter of the measuring pipe 1 are contacted within a prescribed tolerance and are welded so that the cylindrical plates with openings 2a does not float from the inner face of the measuring pipe 1. Regarding this problem, a solving method is disclosed in Patent Document 1, for example.
But in the forming method of the resin lining disclosed in Patent Document 1, there is a problem as described below.
A radial thickness of the resin lining portion 3 is set by providing a prescribed gap x between the cylindrical plate with openings 2a and the measuring pipe 1 at the circumference of the lining locking frame 27 as shown in FIG. 8C. Regarding the gap x, the radial thickness t in the direction perpendicular to the pipe axis is set by a plate thickness t of the belt-like ring plates 2b2, for example.
The resin lining portion 3 is formed mainly by pressure injecting the melted lining resin in the space partitioned by the belt-like ring plates 2b2 from a lot of the openings A of the cylindrical plate with openings 2a. In this time, in order to keep the quality of a uniform strength at each portion of the resin lining portion 3, it is required to form the resin lining portion 3 without including the air bubbles and without remaining residual stress in the lining resin itself caused by the non-uniformity of the pressure at the pressing places.
For the reason, in case of the lining locking frame with a construction disclosed in Patent Document 1, there was a problem that a high level molding technique such as transfer molding or injection molding is required to heat the measuring pipe and the die for mold forming, and in addition, to inpour the lining resin by pressure melting at not less than the prescribed temperature.
In addition, in forming the resin lining according to the electromagnetic flow meter of above-described Patent Document 2, a technique is disclosed to perform lining by trenching a plurality of dovetail grooves extending in the direction of the entire length of the pipe axis along the circumference direction at the inner circumference face of the measuring pipe, so as to prevent the lining resin from peeling off from the measuring pipe.
However, so as to provide a plurality of such dovetail grooves, there is a problem that though it is possible to fabricate the dovetail grooves at the end portions of the measuring pipe, it is difficult to fabricate the dovetail grooves at the central portion of the measuring pipe. In addition, as the lining resin is to be inpoured at the acute portions of the dovetail grooves without including air bubbles, there is a problem after all that a high level molding technique such as transfer molding or injection molding is required.